Flexible coaxial cables are frequently used as transmission lines for radio frequency, microwave frequency, and millimeter wave frequency electromagnetic waves. These high frequency waves are capable of carrying many signals simultaneously. Physical maintenance of the signal path is critical to transmitting the signals from one point to another without distortion (return loss) or attenuation (signal loss). The flexible coaxial cables used have an inner conductor of diameter "d" and an outer conductor (shield) of diameter "D". The inner conductor is typically stranded or solid wire and the outer conductor is typically braided metal wire, helically wrapped metal foil, helically-wrapped round wire, or helically wrapped metal-plated or metal-coated polymer. The ratio of the diameter of the inner and outer conductors and the dielectric constant of the material separating them determines cable impedance and must be maintained within tight tolerances. Any distortions due to denting, crushing, or otherwise introducing a non-concentric relationship will result in higher distortion (return loss) and higher attenuation (signal loss). Also, if the integrity of the outer conductor (shield) is interrupted, energy will escape. Torsional (twisting) force can cause the outer conductor to open resulting in an interrupted signal path. The types of damage (denting, crushing, kinking, twisting) described often occur during installation and use due to the cable being bent over sharp objects, clamped too tightly, struck by another object, twisted, or bent beyond its minimum bend radius.
These types of damage are more likely in flexible cables that use air-spaced dielectric materials, but can also occur in cables using solid dielectrics.
In the past, two main approaches have been used to protect cables from crushing and torsional damage. The first is extra layers over the shield of the cable such as braided wires and/or hard-film wraps such as Kapton.RTM. polyimide and thicker external jackets. These tend to be very stiff. The second approach is the use of external means of providing added protection in the form of flexible conduits. Typical examples would be springs covered with extruded polymers or shrink tubes and flexible metal conduits (armors). The external conduit or ruggedizations such as shown in U.S. Pat. No. 4,731,502, while adding significant crush and/or torque resistance, add significantly to the weight and diameter of the cable.